-your local library may stock "Laws' guide to the Sierra Nevada"
– I love phone apps, especially free ones. I uploaded a music recognition app & googled plant recognition app, leafsnap.com came up. Oh course this may only work if you have cell or wifi service
I have seen brochures at big trees park which have good guides on the local plants. The tour guides are generally very knowledgable. I talked to an older gentleman Who in twenty minutes gave me a tremendous amount of info on trees.
Laws' guide to the Sierra Nevada, not sure of the exact title, is indispensible for common plants, birds, and other wildlife. Beautiful illustrations and brief natural history notes. Everyone should have a copy. Good size for carrying around or including in your day pack. Author is John Muir Laws, most any bookstore should stock it.
It's not free or particularly cheap – but the best guide to plants, insects, any any other life in the area is Sierra Nevada Natural History from the University of California Press http://www.ucpress.edu/book.php?isbn=9780520240964 This is the definitive guide for out area